Objectivity is crucial. Whether giving or receiving critique, leaving space is essential and takes practice. A majority of the time, critique is not personal. (Yes, trolls can happen, but they are relatively rare on decent forums like this.) If someone says they struggle to relate to a narrator, take it with a grain of salt. It isn't that a narrator is poorly written, it might be something as simple as the perspective and perceptions of the narrator are novel and completely alien to the reader's functional context. (e.g. A double empathy autistic reading memoir about a diagnosed sociopath.) Decent critique can highlight this. It is an indicator that it makes a reader pause and think about the context, something effective writing does.
Go in with realistic expectations. Your skill set is not going to be the same as someone else's. Saying this piece was nice is a cop out on both sides. Sure it makes a reader feel nice, but it doesn't offer anything constructive. Vanity post type two diabetes is a very real pitfall. A balanced diet of constructive insight and moderate practice (exercise) is necessary in any type of creative endeavor. A majority of writers are not looking for the efforts of a balanced critique. An even larger portion of writers don't actually know how to critique. This is a statistical reality.
At its most basic, critique comes down to what works and why. What didn't work and why. This takes time and effort to develop, but it makes one a more conscious writer over time. It is one of those hands on things that can teach far more effectively than any text book or course can.
If the why (explanation with examples and/or reasoning) is not present, process the advice with caution because that slides dangerously into the realm of lost objectivity. The fourth wall is not only there to protect the reader, it is there as a shield for the author, as well.
Critique isn't about nice feelings or a quick reply. It is the active engagement of critical thinking skills and articulation of one's observations. Keep it fair, objective, and balanced. If you read something and can't find anything constructive to offer, step away.